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Date: Tue, 28 Jun 2011 20:42:47 +0800
From: Hillf Danton <dhillf@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: [RFC] sched: use trylock in pulling and pushing RT tasks
Hi Steven
When pulling/pushing RT tasks from/to neighbor runqueues, if we canot take
the lock of neighbor RQ at the moment, it maybe a good news since the
neighbors are likely busy in scheduling/balancing, and we could shorten
the exec paths of puller and pusher.
For puller, it is a simple replacement of double_lock_balance with
raw_spin_trylock; for pusher, the manner to find and lock the lowest RQ is
changed a bit for trylock.
Hillf
---
kernel/sched_rt.c | 119 ++++++++++-------------------------------------------
1 files changed, 22 insertions(+), 97 deletions(-)
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index b03cd89..545a2b1 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1178,9 +1178,6 @@ static void put_prev_task_rt(struct rq *rq,
struct task_struct *p)
#ifdef CONFIG_SMP
-/* Only try algorithms three times */
-#define RT_MAX_TRIES 3
-
static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep);
static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
@@ -1306,51 +1303,30 @@ static int find_lowest_rq(struct task_struct *task)
return -1;
}
-/* Will lock the rq it finds */
static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
{
- struct rq *lowest_rq = NULL;
- int tries;
- int cpu;
-
- for (tries = 0; tries < RT_MAX_TRIES; tries++) {
- cpu = find_lowest_rq(task);
+ int cpu = rq->cpu;
+ struct cpumask *lowest_mask = __get_cpu_var(local_cpu_mask);
- if ((cpu == -1) || (cpu == rq->cpu))
- break;
+ if (!lowest_mask ||
+ !cpupri_find(&rq->rd->cpupri, task, lowest_mask) ||
+ /* no more push if we are low enough in prio */
+ cpumask_test_cpu(cpu, lowest_mask))
+ return NULL;
- lowest_rq = cpu_rq(cpu);
-
- /* if the prio of this runqueue changed, try again */
- if (double_lock_balance(rq, lowest_rq)) {
- /*
- * We had to unlock the run queue. In
- * the mean time, task could have
- * migrated already or had its affinity changed.
- * Also make sure that it wasn't scheduled on its rq.
- */
- if (unlikely(task_rq(task) != rq ||
- !cpumask_test_cpu(lowest_rq->cpu,
- &task->cpus_allowed) ||
- task_running(rq, task) ||
- !task->on_rq)) {
-
- raw_spin_unlock(&lowest_rq->lock);
- lowest_rq = NULL;
- break;
- }
- }
+ for_each_cpu(cpu, lowest_mask) {
+ struct rq *lowest_rq = cpu_rq(cpu);
+ if (!raw_spin_trylock(&lowest_rq->lock))
+ continue;
/* If this rq is still suitable use it. */
if (lowest_rq->rt.highest_prio.curr > task->prio)
- break;
-
- /* try again */
- double_unlock_balance(rq, lowest_rq);
- lowest_rq = NULL;
+ return lowest_rq;
+ raw_spin_unlock(&lowest_rq->lock);
+ break;
}
- return lowest_rq;
+ return NULL;
}
static struct task_struct *pick_next_pushable_task(struct rq *rq)
@@ -1390,7 +1366,6 @@ static int push_rt_task(struct rq *rq)
if (!next_task)
return 0;
-retry:
if (unlikely(next_task == rq->curr)) {
WARN_ON(1);
return 0;
@@ -1406,44 +1381,10 @@ retry:
return 0;
}
- /* We might release rq lock */
- get_task_struct(next_task);
-
/* find_lock_lowest_rq locks the rq if found */
lowest_rq = find_lock_lowest_rq(next_task, rq);
- if (!lowest_rq) {
- struct task_struct *task;
- /*
- * find lock_lowest_rq releases rq->lock
- * so it is possible that next_task has migrated.
- *
- * We need to make sure that the task is still on the same
- * run-queue and is also still the next task eligible for
- * pushing.
- */
- task = pick_next_pushable_task(rq);
- if (task_cpu(next_task) == rq->cpu && task == next_task) {
- /*
- * If we get here, the task hasn't moved at all, but
- * it has failed to push. We will not try again,
- * since the other cpus will pull from us when they
- * are ready.
- */
- dequeue_pushable_task(rq, next_task);
- goto out;
- }
-
- if (!task)
- /* No more tasks, just exit */
- goto out;
-
- /*
- * Something has shifted, try again.
- */
- put_task_struct(next_task);
- next_task = task;
- goto retry;
- }
+ if (!lowest_rq)
+ return 0;
deactivate_task(rq, next_task, 0);
set_task_cpu(next_task, lowest_rq->cpu);
@@ -1451,10 +1392,7 @@ retry:
resched_task(lowest_rq->curr);
- double_unlock_balance(rq, lowest_rq);
-
-out:
- put_task_struct(next_task);
+ raw_spin_unlock(&lowest_rq->lock);
return 1;
}
@@ -1481,25 +1419,10 @@ static int pull_rt_task(struct rq *this_rq)
src_rq = cpu_rq(cpu);
- /*
- * Don't bother taking the src_rq->lock if the next highest
- * task is known to be lower-priority than our current task.
- * This may look racy, but if this value is about to go
- * logically higher, the src_rq will push this task away.
- * And if its going logically lower, we do not care
- */
- if (src_rq->rt.highest_prio.next >=
- this_rq->rt.highest_prio.curr)
+ if (!raw_spin_trylock(&src_rq->lock))
continue;
/*
- * We can potentially drop this_rq's lock in
- * double_lock_balance, and another CPU could
- * alter this_rq
- */
- double_lock_balance(this_rq, src_rq);
-
- /*
* Are there still pullable RT tasks?
*/
if (src_rq->rt.rt_nr_running <= 1)
@@ -1529,8 +1452,10 @@ static int pull_rt_task(struct rq *this_rq)
ret = 1;
deactivate_task(src_rq, p, 0);
+ raw_spin_unlock(&src_rq->lock);
set_task_cpu(p, this_cpu);
activate_task(this_rq, p, 0);
+ continue;
/*
* We continue with the search, just in
* case there's an even higher prio task
@@ -1539,7 +1464,7 @@ static int pull_rt_task(struct rq *this_rq)
*/
}
skip:
- double_unlock_balance(this_rq, src_rq);
+ raw_spin_unlock(&src_rq->lock);
}
return ret;
--
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